In recent years, with the increase in telecommunications traffic, high-speed optical transmission systems have been requested. High-speed optical transmission systems preferably use an optical modulator that can modulate light at high speed.
A typical optical modulator includes a modulator that modulates light that is emitted from a light source by using a received data-signal. The modulator is made of, for example, dielectrics, such as LiNbO3 (LN), or semiconductors, such as InP and GaAs. The modulator splits a light emitted from the light source into a first light and a second light, guides the first light to a first waveguide and the second light to a second waveguide, and then superposes a data signal on the first light and a data signal on the second light. When data signals are respectively superposed on the first light and the second light, respective synthesized signal lights are generated and the optical signals are then output from the first waveguide and the second waveguide, respectively.
Being subjected to, for example, a temperature change and a temporal change (hereinafter, “temporal change, etc.”), the phase of a signal light output from the first waveguide and/or the phase of a signal light output from the second waveguide may have changed from their respective target values. Any phase change degrades the waveform of an optical signal output from the modulator. An optical signal that has a degraded waveform decreases the transmission performance of the optical transmission device that transmits the optical signal.
Various technologies are considered to compensate a phase that has changed from a target value due to a temperature change, etc., (hereinafter, “phase degradation”). A well-known technology, for example, involves detecting the difference between the phase of a data signal that is superposed while passing through a waveguide and the phase of the signal light output from the waveguide and then adjusting the bias voltage that is applied to the waveguide in such a manner that the detected phase difference becomes zero. As described above, it is possible to decrease the extent of phase degradation caused by a temperature change, etc., by adjusting the bias voltage applied to the waveguide.    Patent Document 1: Japanese Laid-open Patent Publication No. 2006-251087    Patent Document 2: Japanese Laid-open Patent Publication No. 2010-081287
However, the conventional bias-voltage adjusting technology has a problem in that, if the modulator is made of a semiconductor, the power of the optical signal output from the modulator decreases.
The above problem is described with reference to FIG. 13. FIG. 13 is a graph that illustrates the properties of a Mach-Zehnder optical modulator that is made of a semiconductor. The horizontal axis of FIG. 13 is the bias voltage applied to the semiconductor Mach-Zehnder optical modulator; the vertical axis is the phase and the optical absorption (absorption). A curve 11 of FIG. 13 indicates the phase property of the semiconductor Mach-Zehnder optical modulator; a curve 12 indicates the absorption property of the semiconductor Mach-Zehnder optical modulator.
The phase property curve 11 of FIG. 13 can shift to a curve 13 due to a temperature change, a temporal change, etc. To compensate a change in the phase property, i.e., phase degradation, according to the conventional technology, the bias voltage that is applied to the waveguide of the semiconductor Mach-Zehnder optical modulator is increased from a default value V0 to V1.
The absorption property of the semiconductor Mach-Zehnder optical modulator indicated by the curve 12 increases as the bias voltage increases. In the example of FIG. 13, when the bias voltage is increased from V0 to V1, the absorption increases from A0 to A1. An increase in the absorption causes a light loss through the waveguide, which decreases the power of the optical signal output from the semiconductor Mach-Zehnder optical modulator.